The present invention relates to a quantum key distribution system using a quantum entangled photon pair. More specifically, the present invention relates to a multi-terminal quantum key distribution system which makes it possible to perform quantum key distribution between two given terminals among a plurality of terminals, and which makes it possible to simultaneously perform quantum key distribution between a plurality of sets of two terminals.
A quantum encryption communication system is expected to achieve ultimate secure communication, and it is expected that the quantum encryption communication system will be applied to future high-security information communication systems. Here, in order to achieve safe communication without information leakage, it is necessary for a sender and a receiver to share an encryption key, which is used to encode and decode information, such that the encryption key is not known by a third party, such as an eavesdropper.
The quantum key distribution system is attracting attention as an encryption key distribution system in which ultimate unconditional security is guaranteed by the laws of quantum physics, and recently, research and development of the quantum key distribution system is being actively carried out in order for it to be applied to future high-security information communication systems (refer to, for example, Japanese Patent Application Publication No. JP-A-2012-049890 or Japanese Patent Application Publication No. JP-A-2012-004955).
A practical quantum key distribution system is achieved by using a single photon generating device that generates one photon per pulse, or a quantum entangled photon pair generating device that generates one pair of quantum entangled photons per pulse.
When the single photon generating device is used, the sender generates a single photon and the receiver receives it using a single photon detector. After that, the sender and the receiver share a final encryption key through processes including the exchange of information about a measurement basis, error correction and privacy amplification. Privacy amplification is a technology that appropriately discards and shortens a bit stream that has become partially known to an eavesdropper, and thereby generates a secure secret key in terms of information theory.
On the other hand, when the quantum entangled photon pair generating device is used, the sender and the receiver (hereinafter, these are sometimes collectively referred to as a sender/receiver) respectively receive one of a pair of quantum entangled photons using the single photon detector, and thereby share the encryption key.